1. Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display using the same, and more particularly, to a backlight unit and a liquid crystal display device using the same that can reduce one or more of power consumption, electromagnetic interference, heat generation, noise, and wavy noise.
2. Discussion of the Related Art
The use of flat panel displays has recently been growing exponentially. Among the flat panel displays, a liquid crystal display has been widely used in large-sized digital televisions requiring a thin profile, as well as in small-sized mobile devices requiring low power consumption.
Because liquid crystal cells formed on a liquid crystal display panel of the liquid crystal display do not emit light by themselves, the liquid crystal display requires a backlight unit. The backlight unit is typically positioned at a back surface of the liquid crystal display panel and irradiates light onto the liquid crystal display panel. The backlight unit greatly affects the performance of the liquid crystal display. For example, the backlight unit may greatly affect, among other things, the weight, design, lifespan, and power consumption of the liquid crystal display, as well as such image quality characteristics of the liquid crystal display as the color reproduction, maximum brightness, a contrast ratio, white uniformity, and a color temperature.
Existing backlight units widely used a cold cathode fluorescent lamp (CCFL) or an external electrode fluorescent lamp (EEFL) as their light source. However, the fluorescent lamps have a relatively large size, high power consumption, and low brightness. Hence, some of the recent backlight units use a light emitting diode (LED) as the light source.
The LED is more eco-friendly and has a faster response time than the fluorescent lamps. Further, color reproduction of the LED is as high as about 80 to 100%. The backlight unit using the LED as the light source (hereinafter referred to as the “LED backlight unit”) has an advantage in that it can readily adjust the luminance and color temperature of the LED backlight unit by adjusting the amount of light emitted by the LED.
The LED backlight unit includes a plurality of LED arrays and a light source driving device. The LED arrays include a plurality of LED channels, and each LED channel is configured as a plurality of LEDs connected in series to one another. The LEDs are driven by the light source driving device. Examples of a method for controlling the luminance of the LEDs using the light source driving device include a pulse width modulation (PWM) control method and a pulse amplitude modulation (PAM) control method.
As shown in FIG. 1, the PWM control method uniformly fixes the amplitude (indicated by the current in a graph shown in FIG. 1) of a pulse and changes the width of the pulse every predetermined period T, thereby controlling the luminance of the LEDs. The PWM control method can perform both a backlight scanning drive and a local dimming drive capable of adjusting the brightness of the LED backlight unit at a desired position by controlling the luminance of the LEDs. The backlight scanning drive means a driving method for sequentially turning on and off the light sources of the LED backlight unit along a data scan direction of the liquid crystal display panel. The local dimming drive means a backlight dimming method for dividing the LEDs into a plurality of blocks and adjusting a dimming value of each block, so as to locally control the luminance of a display surface in one frame period. Because the PWM control method controls the luminance of the LEDs using the pulse width, it is relatively easy to achieve the low luminance.
On the other hand, as shown in FIG. 2, the PAM control method uniformly fixes the width of a pulse and changes only the amplitude of the pulse every predetermined period T, thereby controlling the luminance of the LEDs. In the PAM control method, it is impossible to perform the backlight scanning drive and the local dimming drive. Further, because the PAM control method controls the luminance of the LEDs using the amplitude of the pulse, it is difficult to achieve the low luminance.
The related art light source driving device controlled the luminance of the LEDs using the PWM control method for the backlight scanning drive and the local dimming drive. However, the PWM control method has the following problems.
First, as shown in FIG. 3A, in the PWM control method, the forward voltage VF between both terminals of the LED increases in proportion to the forward current IF flowing in the LED. Because the forward current IF is fixed to a predetermined maximum value in the PWM control method, the forward voltage VF in the PWM control method is greater than that in the PAM control method. Thus, as shown in FIG. 3B, the power consumption, electromagnetic interference (EMI), and heat generation in the PWM control method are greater than those in the PAM control method.
Second, the amplitude of an input/output current in the PWM control method is greater than that in the PAM control method. Hence, more noise is generated by a repetition of compression and expansion resulting from a current flowing in a circuit part.
Third, because the PWM control method turns on and off the LEDs so as to control the luminance of the LEDs, a wavy noise is generated in a display image due to the turn-on and the turn-off of light energy.